Current piston ring and piston assemblies in internal combustion engines have very high friction. There are substantial fuel economy gains available if it is possible to reduce this operating friction. Calculation indicates that piston ring and piston assembly friction would be available if the piston rings and pistons functioned as optimally designed bearings. Current ring and piston assemblies operate with very thin oil films, largely because the ring assemblies function with oil scrapers which scrape the oil layer on the cylinder wall down to extremely thin values. This results in very high ring friction and high piston skirt friction. Moreover, the pistons which have evolved over time are not designed to utilize the squeeze film and hydrodynamic lubrication effects efficiently. This is partly because of the scraping of the cylinder wall oil layer from the rings, and also partly because of problems with differential expansion of various areas of the piston.
It is worthwhile to review the fundamentals required for design of a low friction piston ring and piston assembly. The friction that occurs between parts separated by a full oil film is much lower than the friction that occurs in boundary lubrication. Since the film friction for a full film is inverse with film thickness, there is a strong incentive to produce the thickest possible oil films between sliding parts.
It is well documented in the literature that convergent wedge-shaped films generate significant pressures. The equation for oil wedges in Reynold's equation, which is explained thoroughly in a number of textbooks and references (i.e. Chapter 3, page 3-4 in Standard Handbook of Lubrication Engineering, O'Connor and Boyd, McGraw-Hill, New York, 1968). The application of Reynold's equation is well understood in cases that involve fully convergent wedges. In cases involving geometries with both convergent and divergent sections, Reynold's equation is easy to apply for heavily loaded cases. Hydrodynamic lubrication has been exhaustively treated in the literature. It is an area of mechanical engineering where exact mathematical equations are known to work within any reasonable experimental error.
In the engine context, an additional important load bearing potential exists because of the cyclical nature of the loads. If two surfaces are separated by an oil layer and a force is applied pushing the surfaces together, the oil layer which must be squeezed out between the surfaces generates a force which resists the approach of the surfaces. This is the squeeze film effect. In a piston, the load on the skirts fully reverses during every cycle so that it is possible to take advantage of squeeze film effects if the piston skirts are supplied with enough oil. The magnitude of the squeeze film effect varies greatly with changes in the geometry of the approaching surfaces. The geometric effects are exactly calculable for simple cases (and always calcuable using the calculus if geometry and viscosity are known). A sense of the critical nature of the squeeze film effect can be achieved by looking at the equations for the squeeze film effect for planar surfaces. For planar surfaces approaching each other, the formulas for resistance force W and time to collapse a set distance under a set load delta T have the general formulas: ##EQU1## (where .mu. is viscosity, W is resisting force, .DELTA.t is time to collapse film from distance h.sub.2 to h.sub.1, k.sub.1 and k.sub.2 are geometrical coefficients, h is film thickness, h.sub.2 is film thickness at t.sub.2 and h.sub.1 is film thickness at t.sub.1.). For cylindrical surfaces approaching each other, squeeze film force W and time to collapse a set distance under a set load are similarly dependant on the match of radius of curvature of the two surfaces.
The convergent angles and geometrical relations of hydrodynamic film physics involve critical geometrical issues. The heavy loads occuring in engines invariably deform the parts in a way which affects the film forming geometry and the friction. The film thicknesses and geometrical relations often required for optimal fullfilm physics require geometrical precision not reasonably obtainable in production. Moreover, heavy loads produce deformations such that the oil film forming geometry varies from the geometries which would occur if the engine parts were infinitely stiff. Differential expansion of parts also alters film forming geometry. It is a purpose of the present invention to present designs of piston rings and pistons where the parts are shaped and cooled so that elastic deformations automatically adjust geometry to maximize oil film stability and minimize friction, even in the presence of the deformation and geometrical imperfections encountered in real engines. Optimization of flexibility of the rings and the piston skirts permits fine scale adjustment of film forming geometry which is impossible with production tolerances alone, particularly when differential expansion of parts is considered.
The piston assembly consists of an internal heat piped cooling arrangement which makes the piston effectively isothermal (and particularly makes the skirts isothermal) to permit optimization of geometry. The piston skirts are designed to be flexible with respect to circumferential out-of-round of the cylinder wall and to maximize squeeze film effects. These skirts are shaped with centrally pivoted pivoted pad slider surfaces pivoting on the wrist pin to produce the low friction high load bearing characteristics of pivoted pad sliders. On both the top and bottom sections of the piston skirt, a barrel section at large radius is provided to act as a sled runner and to catch piston rock in squeeze film mode. The barrel sections assure optimal pivoting of the piston skirt. Heat transfer from the piston crown via the heat piped section occurs through the piston skirt surfaces, where conductance through the oil film transfers the heat to the cylinder walls.
The rings are flexible piston rings mounted in circumferential compression with end-gap springs, and are arranged to conform to out-of-round of the cylinder. Ring conformance is sufficiently rapid to accommodate variations in cylinder out-of-round as the piston moves axially. The ring pack is arranged with several rings engaging the cylinder wall in a manner which uses cylinder on flat plate fluid mechanics, which has been well documented. The arrangement of spring forces, gas actuating forces, and radii of curvature produce a situation where the bottom ring, called the oil control ring, is always fully flooded and the other rings are operated in a partially starved but fully hydrodynamic condition. The rings should have essentially zero wear, and are characterized by extremely low friction. The ring assembly does not have any conventional scraper, and functions in a manner permitting fully flooded lubrication of the piston skirts. In the place in the piston where an oil scraper might ordinarily be is a groove functioning as an oil reservoir to assure that the piston skirt is fully flooded and the ring pack is maintained in the full film lubrication regime.